27 



exit end of the tube. It is important that the oxygen should 

 be perfectly dry, otherwise the quantity of ozone formed will be 

 very small. 



115. Galvanometer. 

 E. 60. 1874. 



The action of this instrument depends upon the fact 

 that a magnetic needle, freely suspended at its centre, and 

 placed in the neighbourhood of a wire or other conductor 

 along which a current of electricity is passing, tends to place 

 itself at right angles to the line of the current. In practice 

 two parallel needles are mounted the one above the other 

 upon the same pivot, with their poles in contrary directions, 

 so that the action of the earth's magnetism upon them shall be 

 reduced to a minimum. The lower needle is then adjusted 

 within a coil of silk-covered copper wire, and the upper one 

 above a cardboard disk on which is marked an arc graduated 

 into degrees. On a current of electricity traversing the coil of 

 wire, even though it be the feeble current produced by a slight 

 variation of temperature on the face of a thermopile, the motion 

 of the upper needle at once indicates the existence, and to some 

 extent the force, of the current. 



116. Thermopile. 

 E. 61. 1874. 



An instrument for producing feeble currents of electricity by 

 warming at their points of junction bars of bismuth and 

 antimony soldered together. A number of couples of this 

 kind are packed in a brass case, so that half the joints may 

 be on one side and half on the other. The slightest variation 

 of temperature between the two sets of joints is enough to 

 excite a current of electricity, which may by means of wires 

 attached to the terminal bars be conducted to a galvanometer, 

 and measured. The heat given off by the human body at a 

 distance of 10 yards is sufficient to affect a delicate instrument. 



117. Darnell's hygrometer. 

 E. 62. 1874. 



This is an instrument for determining the " dew-point," or 

 the temperature at which the aqueous vapour contained in 

 the air begins to be condensed. It consists of a glass siphon 

 connecting two bulbs. One of the bulbs is blackened, and 

 contains ether, into which dips a small thermometer for the 

 determination of the temperature of the liquid. The instrument 

 is mounted on a stand bearing another thermometer to take 

 the temperature of the surrounding air. The empty bulb is 

 covered with muslin tied tightly round it. To use the hygro- 

 meter, a little ether is poured upon the muslin, and the cold 

 produced by its rapid evaporation condenses the ether vapour 

 inside the empty bulb as fast as it is formed in the blackened bulb. 

 By this action the temperature in the blackened bulb is lowered. 

 As soon as the dew-point is reached, aqueous vapour from the 



